As a power transmission system like this, Patent Literature 1 discloses a configuration that a first speed changer and a second speed changer are constructed from planetary gear mechanisms and that ring gears thereof are coupled to each other in a manner not permitting relative rotation. When the ring gear engages with the planetary gear, a thrust force acts on the ring gear in the axial direction. Nevertheless, Patent Literature 1 does not mention this point at all.
In contrast, Patent Literature 2 describes that a thrust force generated between a large diameter gear of a double pinion and an idler gear and a thrust force generated between a small diameter gear of a double pinion and a driving output gear canceled out with each other through a thrust bearing arranged between the double pinion of a first speed changer and the double pinion of a second speed changer which are located opposite to each other.
Further, a power transmission system described in Patent Literature 3 describes a thrust force generated in a large diameter gear of a double pinion of a planetary gear mechanism and a thrust force generated in a small diameter gear of the double pinion are canceled out with each other in the inside of the double pinion.